Medulloblastoma is one of the most common malignant brain tumors of children. To approach the problem of effective therapy, the development and progression of human medulloblastoma must first be understood. A hypothesis is that shedding of structurally distinct tumor gangliosides influences the biology of medulloblastoma. Human medulloblastoma cells have been shown to shed gangliosides in vitro, and in vivo into the cerebrospinal fluid (CSF), and tumor gangliosides were previously established to be potent inhibitors of the immune response. Based on these findings, the proposal is to establish the biological significance of shed medulloblastoma gangliosides by determining the influence of ganglioside shedding on tumor formation in experimental models. The applicant will also determine if specific ceramide species of GD3 are shed by medulloblastoma and thus are candidate tumor markers. To test the hypothesis, the plan is to first define unique ceramide species present in the cerebrospinal fluid of patients with medulloblastoma, using the newly developed matrix-assisted laser desorption/ionization mass spectrometry method (MALDI-MS) to detect specific GD3 ceramide species in small quantities of CSF. Secondly, the intent is to document the transfer and target cell binding of gangliosides shed by medulloblastoma tumor cells, using a new in vitro model which mimics the in vivo tumor cell micro environment, and determine the effect of binding on target cell function. Third, the plan is to develop a reliable method for downregulating tumor cell ganglioside synthesis (and therefore shedding) utilizing either glucosylceramide synthase inhibitors (PDMP/PPPP) or antisense sequences complementary to the glucosylceramide synthase gene. Abrogation of tumor ganglioside synthesis and shedding will allow the assessment of the biological activity of specific medulloblastoma tumor gangliosides and their role in enhancing tumor formation, and will be essential to the accomplishment of the final aim, investigation of the influence of the inhibition of tumor ganglioside synthesis and shedding on tumor formation in two experimental animal models. Taken together, these studies may increase our understanding of the role of medulloblastoma gangliosides in tumor development. The proposed project should increase our knowledge of the biology of medulloblastoma and facilitate development of novel therapeutic approaches.